1. Field of the Invention
This invention relates to the chemical analysis of substances, and more particularly, to a method for the precise metering of biological fluids onto test elements.
2. State of the Prior Art
A number of automated systems have been developed for performing quantitative chemical analyses of fluid samples. Most of the commercially-available systems utilize liquid reagents and require analyzer equipment having intricate solution handling and transport capabilities. Recent developments, however, have provided test elements in essentially planar, dry form which can be loaded into a cartridge for use in an analyzer. In the use of such an analyzer, a test element from a cartridge is fed into a metering station where a predetermined amount of sample fluid is deposited on the test element. After an incubation period, the element is moved to a read station where a change in the test element is measured, the amount of change being proportional to a particular analyte in the fluid. The test element is used only once and is discarded after the reading has been taken. An analyzer for use with such test elements is disclosed in commonly-assigned U.S. Pat. No. 4,152,390.
Test elements of the type described above are adapted to function with very small quantities of sample fluid. For example, test elements for performing colorimetric analyses can produce a measurable response with only 10 microliters of sample fluid, and elements for performing potentiometric analyses are operable with 10 microliters of sample fluid and 10 microliters of reference fluid. The volume of fluid supplied to the elements should preferably not vary more than 5% from a selected value to achieve desirable test results. Thus, there is a problem in providing a metering device which can deliver precise micro quantities of fluid, in spite of variations in the physical properties of the fluid and the test elements. Moreover, in high-throughput analyzers, the metering device must be capable of repeatedly and accurately dispensing such quantities of fluid onto the test elements as they are sequentially moved into a metering station.
In European Patent Application No. 400940.3, Pub. No. 0042337, published Dec. 23, 1981, there is disclosed a method and apparatus for the precise metering of biological fluids onto generally planar test elements, or analysis slides. The apparatus comprises a dispenser which is adapted to pick up a disposable metering tip, aspirate fluid into the tip, meter a predetermined amount of fluid from the tip onto an analysis slide, and eject the tip after the metering operation. In the use of such metering apparatus in a high-throughput analyzer, the metering device must be rapidly moved from the aspiration station to the metering position where the fluid is deposited on an analysis slide; best results are obtained if the metering tip is moved closely adjacent the slide, preferably within a space between about 0.030 cm and about 0.15 cm from the slide. However, a problem occurs if a tip containing fluid is rapidly moved toward a slide and then stopped suddenly in a position closely adjacent the slide. When the metering tip is stopped suddenly, fluid within the metering tip tends to momentarily protrude from the tip due to the effect of inertia on the fluid; if the tip is close enough to the slide when it is stopped, the protruding fluid will contact the slide and a condition known as "pre-spot" occurs in which a certain amount of fluid is deposited on the slide. Such a condition is undesirable, since it tends to produce uneven distribution of fluid on the slide which causes inaccurate test results.